GB/T 3-1997 Metric Thread Ends and Chamfers Standard

mennessä | joulu 24, 2025 | Tekninen dokumentaatio ja viitteet

Introduction to the Standard

GB/T 3-1997 specifies dimensions for run-outs, undercuts, shoulders, and chamfers of general-purpose metric screw threads, ensuring reliable fastening connections. It applies to threads with pitches from 0.2 mm to 6 mm, promoting consistency in manufacturing and assembly across mechanical industries.

Equivalent to ISO 3508:1976 and ISO 4755:1983, this standard provides guidelines for external and internal threads, facilitating interchangeability and reducing assembly issues. It is essential for precision engineering, where thread terminations affect strength and fit.

  • Scope: Covers standard metric threads for general fastening.
  • Importance: Prevents stress concentrations and ensures proper mating.
  • Applicability: Extends to similar thread types like transition fits.

External Thread Ends and Shoulders

External thread run-outs (x) and shoulders (a) are defined to avoid sharp edges and ensure smooth assembly. Dimensions vary by pitch, with general, short, and long options for different structural needs. The bottom radius of run-outs must not be less than the minimum fillet radius for full threads.

Pitch P (mm)Run-out x (max)Shoulder a (max)
GeneralShortGeneralLongShort
0.20.50.250.60.80.4
0.250.60.30.7510.5
0.30.750.40.91.20.6
0.350.90.451.051.40.7
0.410.51.21.60.8
0.451.10.61.351.80.9
0.51.250.71.521
0.61.50.751.82.41.2
0.71.750.92.12.81.4
0.751.912.2531.5
0.8212.43.21.6
12.51.25342
1.253.21.6452.5
1.53.81.94.563
1.754.42.25.373.5
252.5684
2.56.33.27.5105
37.53.89126
3.594.510.5147
410512168
4.5115.513.5189
512.56.3152010
5.514716.52211
6157.5182412
Reference≈2.5 P≈1.25 P≈3 P=4 P=2 P

Notes: Prefer general lengths; use short for constrained structures, long for B or C grade products. Dimensions in mm.

External Thread Undercuts

Undercuts (g1, g2) relieve stress at thread terminations, with diameters (dg) and radii (r) specified. The transition angle α is at least 30° to avoid sharp corners.

Pitch P (mm)g2 (max)g1 (min)dgr ≈
0.250.750.4d – 0.40.12
0.30.90.5d – 0.50.16
0.351.050.6d – 0.60.16
0.41.20.6d – 0.70.2
0.451.350.7d – 0.70.2
0.51.50.8d – 0.80.2
0.61.80.9d – 10.4
0.72.11.1d – 1.10.4
0.752.251.2d – 1.20.4
0.82.41.3d – 1.30.4
131.6d – 1.60.6
1.253.752d – 20.6
1.54.52.5d – 2.30.8
1.755.253d – 2.61
263.4d – 31
2.57.54.4d – 3.61.2
395.2d – 4.41.6
3.510.56.2d – 51.6
4127d – 5.72
4.513.58d – 6.42.5
5159d – 72.5
5.517.511d – 7.73.2
61811d – 8.33.2
Reference≈3 P–––

Notes: d is nominal diameter; dg tolerance h13 (>3 mm) or h12 (≤3 mm). Use for stress relief in high-load applications.

External Thread Chamfers

Chamfers at thread starts are typically 45°, but 60° or 30° allowed; depth ≥ thread height. For rolled threads, incomplete length ≤ 2P to maintain integrity.

Internal Thread Ends and Shoulders

Internal run-outs (X) and shoulders (A) accommodate mating threads, with general and short options. Prefer general; use long for chip clearance, short for constraints.

Pitch P (mm)Run-out X (max)Shoulder A
–GeneralLong
GeneralShort
0.20.80.41.21.6
0.2510.51.52
0.31.20.61.82.4
0.351.40.72.22.8
0.41.60.82.53.2
0.451.80.92.83.6
0.52134
0.62.41.23.24.8
0.72.81.43.55.6
0.7531.53.86
0.83.21.646.4
14258
1.2552.5610
1.563712
1.7573.5914
2841016
2.51051218
31261422
3.51471624
41681826
4.51892129
520102332
5.522112535
624122838
Reference=4P=2P≈6~5P≈8~6.5P

Notes: No short shoulder specified; dimensions in mm for optimal chip removal.

Internal Thread Undercuts

Internal undercuts (G1) provide relief, with diameters (Dg) and radii (R). Short undercuts for limited spaces; Dg tolerance H13.

Pitch P (mm)G1DgR ≈
GeneralShort
0.521D + 0.30.2
0.62.41.2D + 0.30.3
0.72.81.4D + 0.30.4
0.7531.5D + 0.30.4
0.83.21.6D + 0.30.4
142D + 0.50.5
1.2552.5D + 0.50.6
1.563D + 0.50.8
1.7573.5D + 0.50.9
284D + 0.51
2.5105D + 0.51.2
3126D + 0.51.5
3.5147D + 0.51.8
4168D + 0.52
4.5189D + 0.52.2
52010D + 0.52.5
5.52211D + 0.52.8
62412D + 0.53
Reference=4P=2P–≈0.5 P

Notes: Short undercuts for limited spaces; D nominal diameter, Dg tolerance H13; dimensions in mm.

Internal Thread Chamfers

Entry chamfers are typically 120°, or 90°; diameter (1.05~1)D for easy thread engagement.

Application Guidelines and Considerations

Select dimensions based on pitch and application; general preferred for standard use, short for space limits, long for enhanced performance in grades B/C. Ensure chamfers prevent burrs and undercuts reduce stress risers.

  • Use for metric threads in machinery and fasteners.
  • Verify with ISO equivalents for international compliance.
  • Avoid sharp transitions to minimize fatigue.
  • Apply in rolled threads with incomplete length control.
  • Integrate in CAD for precise modeling.

These practices improve thread durability and assembly efficiency.

Usein kysytyt kysymykset (UKK)

What is the purpose of thread run-outs?

Run-outs smoothly terminate threads, reducing stress concentrations and facilitating assembly without damaging mating parts.

When to use short undercuts?

Short undercuts are for structurally constrained applications where space is limited, but ensure they meet load requirements.

How does pitch affect shoulder dimensions?

Larger pitches require proportionally larger shoulders (e.g., 3P general) to provide adequate bearing surface and strength.

What chamfer angle is recommended?

45° for external starts, 120° for internal entries; alternatives like 60° or 90° based on manufacturing ease.

Is this standard compatible with ISO?

Yes, equivalent to ISO 3508 and 4755, ensuring global interoperability in thread designs.

What tolerance applies to dg?

h13 for d > 3 mm, h12 for d ≤ 3 mm, maintaining precision in undercut diameters.